The present invention relates to the detection of printing media such as a sheet of paper used on a printing device such as a printer or a plotter, and more particularly to the error correction of print positions.
On a conventional large-format printer or plotter, an operator usually sets a printing medium on the device and, then, a medium sensor provided on a print carriage detects the width of the medium and takes it into the device to detect its leading edge at a predetermined position. In that case, as shown in FIG. 3(a), the medium sensor is configured to detect the difference in reflectance between a platen 20 (FIG. 1) and a medium 14. In this configuration, a large sensor spot, 2-3 mm in diameter, is used to detect any medium.
In another configuration, a sensor is provided right above the medium 14 to detect an object directly as shown in FIG. 3(b). This configuration allows a light to be detected right above the medium 14, because a diffused light on the medium, which is a light emitted by an LED 31 and diffused on the medium 14, is detected via a lens 32 and a light-shielding cylinder 33. Therefore, this configuration minimizes the effect of the paper condition and allows to reduce the spot diameter down to about 1 mm, thus enabling high-accuracy medium detection.
Although a sensor with the configuration shown in FIG. 3(a), which detects a light reflected on the printing medium, detects most media, its large sensor-spot diameter decreases the accuracy of the reading position.
In contrast, although a sensor with the configuration shown in FIG. 3(b), which has a small sensor spot, reduces the unevenness in reading positions to 0.5 mm or smaller, it cannot detect a medium, such as a clear film, through which light fully transmits.
Therefore, it is an object of the present invention to provide a medium detecting method and a medium detecting device, as well as a printer, that can read a wider range of printing media with high accuracy.
It is another object of the present invention to provide a printer capable of determining the type of a printing medium.
A medium detecting method according to the present invention is for use in a printer wherein the method detects an edge position of a printing medium while scanning the printing medium, the method comprising the steps of scanning a relatively-low-transmittance reference medium with a first medium sensor and a second medium sensor each to detect an edge position of the reference medium, the first medium sensor being capable of detecting a relatively-low-transmittance printing medium with high accuracy, the second medium sensor being capable of detecting a high-transmittance medium with low accuracy; calculating a difference between the edge positions as a correction value; and if, when detecting any given printing medium, it is difficult for the first medium sensor to detect the printing medium, the second medium sensor is used to detect the medium to correct a resulting edge position with the correction value.
This invention allows a medium (edge position) to be detected with high accuracy regardless of the type of medium.
To implement the method described above, a medium detecting device according to the present invention is for use in a printer wherein the device detects an edge position of a printing medium while scanning the printing medium, the device comprising a first medium sensor capable of detecting a relatively-low-transmittance printing medium with high accuracy; a second medium sensor capable of detecting a high-transmittance medium with low accuracy; means for scanning a relatively-low-transmittance reference medium with the first medium sensor and the second medium sensor each to detect an edge position of the reference medium; means for calculating a difference between the detected edge positions; means for storing the difference as a correction value in a non-volatile manner; and correction means for detecting the medium with the second medium sensor when it is difficult to detect the printing medium with the first medium sensor and for correcting a resulting edge position with the correction value.
Preferably, this medium detecting device further comprises a medium detection unit that contains, in one housing 46, a light source that emits light onto the printing medium obliquely; a first photo-sensor that detects diffused light of the printing medium from right above the printing medium and that acts as the first medium sensor, and a second photo-sensor that accepts a light from the light source reflected upward and obliquely on the printing medium and that acts as the second medium sensor. Installing this medium detection unit on a print carriage eliminates the need for providing separate special scanning means. In addition, the moving position of the medium sensor may be identified with an existing linear scale.
The medium detection device may further comprise medium determination means for calculating, for any given printing medium, a first driving level of the light source for outputting a predetermined sensor output amount of the first photo-sensor and a second driving level of the light source for outputting a predetermined sensor output amount of the second photo-sensor and, based on a difference between the two driving levels, for determining a type of the printing medium.
In addition, based on the determination result, the device may further comprise a printing mode setting means for setting a print mode according to the type of the determined printing medium.